Two Early Cretaceous Fossils Document Transitional Stages in Alvarersaurian Dinosaur Evolution

Highly specialized animals are often difficult to place phylogenetically. The Late Cretaceous members of Alvarersauria represent such an example, having been posited as members of various theropod lineages, including birds [1-11. A 70-million-year ghost lineage exists between them and the Late Jurassic putative alvarersaurian Haplocheirus [12], which preserves so few derived features that its membership in Alvarersauria has recently been questioned [13]. If Haplocheirus is indeed an alvarersaurian, then the 70-million-year gap between Haplocheirus and other alvarersaurians represents the longest temporal hiatus within the fossil record of any theropod subgroup [14]. Here we report two new alvarersaurians from the Early Cretaceous of Western China that document successive, transitional stages in alvarersaurian evolution. They provide further support for Haplocheirus as an alvarersaurian and for alvarersaurians as basal maniraptorans. Furthermore, they suggest that the early biogeographic history of the Alvarersauria involved dispersals from Asia to other continents. The new specimens are temporally, morphologically, and functionally intermediate between Haplocheirus and other known alvarersaurians and provide a striking example of the evolutionary transition from a typical theropod forelimb configuration (i.e., the relatively long arm and three-digit grasping hand of typical tetanuran form in early-branching alvarersaurians) to a highly specialized one (i.e., the highly modified and shortened arm and one-digit digging hand of Late Cretaceous parvicursorines such as Linhenykus [1, 15]). Comprehensive analyses incorporating data from these new finds show that the specialized alvarersaurian forelimb morphology evolved slowly and in a mosaic fashion during the Cretaceous.